Hayabusa2# mission target 1998 KY26 preview: a small optically bright rapid rotator

Understanding the physical characteristics of small bodies in the Solar System is crucial for refining models of their formation and evolution. Although several kilometre-sized asteroids have already been visited by spacecraft, 1998 KY26 will be the first asteroid in the decametre range to be explored in situ. Its diameter and spin period place it above the spin barrier, suggesting that its formation and properties may differ from those of previously visited bodies. However, the key physical characteristics of 1998 KY26 remain poorly understood. We conducted a photometric observing campaign during 1998 KY26’s close approach to Earth in 2024. Our observations revealed that 1998 KY26 has a high optical albedo and colours consistent with an Xe-type taxonomy. We also determined its spin period to be (5.3516±0.0001) minutes—twice as fast as previously reported. Using lightcurve inversion methods, we derived a retrograde pole solution and constructed convex and non-convex shape models. By integrating these models with Goldstone radar data from 1998, we estimated 1998 KY26’s diameter to be (11±2) m, about one-third of the original radar-based results. The derived cohesive strength levels necessary to keep the structure intact, which is less than 20 Pa, suggest a possibility of the asteroid’s rubble pile structure, though this finding does not rule out its monolithic structure. We found that it will be possible to validate these results with future JWST observations. Our comprehensive characterisation of 1998 KY26 can inform the planning of the Hayabusa2# spacecraft’s rendezvous encounter, scheduled for July 2031. Additionally, it provides valuable insights into the non-gravitational forces acting on small Solar System bodies, enhancing our understanding of their orbital evolution which could shed light on the nature of dark comets.